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Old Apr 23, 2005, 5:57 PM   #1
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Is there a battery tester on the market that will test the mAh of a nihm battery? I know the Lacrosse bc-900 charger will do that while it is charging the batteries. I am looking for a tester only to show the mAh level. Thank you for your replies.

Dan
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Old Apr 23, 2005, 6:07 PM   #2
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Dan:

The general consensus is that the only way to accurately measure a battery's capacity in mAh is to fully charge it, then fully discharge it at a known discharge rate (i.e., a specific current draw).

This is how this feature works with the new Lacross BC-900 (it fully charges a battery, discharges it to determine it's capacity, then fully charges it again).


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Old Apr 23, 2005, 6:13 PM   #3
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Jim, Thank you for the imformation.

Dan
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Old May 27, 2005, 6:01 AM   #4
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Hi

Batteries are all rated in ma/hr, that is it will pass this stated current for 1 hour. It is simple to make a tester, AA battery holder a calculated wire wound resistor or better still a lamp of the stated voltage as you can see how the lamp glows. Calculate the current flow by OHMS LAW. I have tried this NIMH batteries and it works great.
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Old May 28, 2005, 6:29 AM   #5
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From one geriatric to another, Hi..I understand ohms law, but dont quite understand your method. Can you explain?

Fully charged NiHh cells have an open circuit voltage of about 1.4v, on load is is 1.2v.

Now as you suggest using a lamp of "stated" voltage & ohms law, how does one calculate the current?

i.e. I = E/R...so what is the E, & what is the R ?

Does the current with a light bulb remain constant over time for the calculation of the mah?

I'd be glad to have your comments.
Regards from NZ..


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Old Jun 2, 2005, 5:39 AM   #6
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HI

Here is the full ohms law.

We have 4 parts too this. VOLTS AMPS RESISTANCE WATTS

VOLTS = W/A A*R sqr(W*R)

AMPS = W/ V V/ R sqr(W/R)

RESISTANCE = W/A V/A V^2/ W

WATTS = V *A V^2/ R A^2 * R

Using 4 AA nimh bateries in a holder, as you say gives us on load 4.8 volts.

The wattage is 11.04 watts rounded down to 11 watts

The resistance is 2.08 ohms rounded down to 2 ohms.
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Old Jun 2, 2005, 5:47 AM   #7
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Seem like I ran out of space

I do not know all the new bulbs (cars) at the moment. You will not get an exact duplication for yor requirements.

But a 6v 2a gives us 12w at a resistance of 3 ohms. The bulb will not burn at 100% but it will give you a reading. If you have an ammeter for DC put it in circuit and check the current flow. Remember NIMH`s maintain the voltage then suddenly drop off.
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Old Jun 19, 2005, 2:55 AM   #8
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Todays unfortunatemistake, left a bit off of R = W/A. It should be R = W/A^2

Black mark sir.
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Old Jun 19, 2005, 9:43 PM   #9
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Baz wrote:
Quote:
Does the current with a light bulb remain constant over time for the calculation of the mah?

As NiMH cells maintain a fairly constant voltage until nearly discharged, and what we want to calculate is the useful mAH, by the time the current starts to fall of due to voltage decrease, the useful capacity of the cell has been reached. Any small variations are somewhat diminished by the temperature coefficient of resistance of the lamp filament. So for all practical purposes the current can be considered constant.

Matching the measured mAH value to the nominal value may be difficult, though, as one needs to know the mfr's test method. Some cells' values are based on ten-hour rate, while others' may be based on 1 or even 20. The same cel may yeild wildly different mAH, depending on how it is tested.

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Old Jun 19, 2005, 11:44 PM   #10
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dekelsey61 wrote:
Quote:
Is there a battery tester on the market that will test the mAh of a nihm battery? I know the Lacrosse bc-900 charger will do that while it is charging the batteries. I am looking for a tester only to show the mAh level. Thank you for your replies.

Dan
In this thread, the first reply is the best.

There isn't aneasyway to determine how much charge is left in a Ni-MH battery, because how much charge you'll get out will depend on how fast you discharge it (i.e., what current you draw from it), how it was last charged, the ambient temperature, how much time it gets to recover when in intermittent use, and other factors as well.

All the methods described above make gross assumptions about what stays constant and what doesn't. In particular, methods using light bulbs and related to their nominal rating are going to be wildly in error, because a light bulb is a hot resistance element that generates lots of heat (plus a little light as a useful by-product). The resistance of any such element is strongly dependent on its temperature, so a light bulb filament's resistance when it's glowing brightly is very different from when it's glowing just a little.

All this has been much discussed in these forums before, so a search for battery testing references in the archives would prove fruitful.

What you can do is compare the performance of a cell with a known fully charged, healthysimilar cell by measuring its voltage on discharge through a standard resistor (which should stay cool throughout). This is most easily done using the cheapest digital multimeter you can find with a built in 'battery tester' designed for AA cells. This still won't tell you the performance you'll get under real service conditions.

Good luck!
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